Was wondering why so much difference in the automatic and manual hp / tq ratings of the TPI 5 liter engine? Why is the auto rated @ so much less? They aren't rating them at the wheels are they? Maybe at the output shaft of the transmission?
John C.

 

They are all "rated" at the crank, same as any other factory motor "ratings".

The reason is, in some years, they got different cams. Some years but not all.

 

Here;
https://www.thirdgen.org/1988-pontiac-firebird
It shows two different hp and tq ratings for the same year model LB9 305ci. The auto cars being (25hp) less. The only difference being auto or manual transmission, and rear gears. Auto trans cars came with different cams than manual trans cars in the same production year?
John C.

 

That's correct. From about 86 to 90 the difference was primarily the cam. The auto trans 305 TPI cars got the "peanut cam" (really small- same cam as in the 305 TBI motors). The 5 speed cars got the good-guy cam, same one used in the big brother 350 TPI engine. (The 350 TPI always got the bigger cam in all years even though it was only offered with an auto trans from the factory)

Around 1990 they changed it around and gave all the TPI engines the good cam but restricted the 305/auto cars with a smaller exhaust system. But there were exceptions on these later 3rd gens like cars that got the dual cat exhaust option, etc.

 

It's about as complex as when they tried building the turboencabulator

http://en.wikipedia.org/wiki/Turboencabulator